The present invention relates to correction of streak unevenness that has developed in an image and particularly to an image correction method and an image correction device capable of restricting occurrence of ringing and preventing removal of a necessary subject to achieve correction of streak unevenness.
A radiographic image detector has been conventionally used to produce diagnostic images in medicine or to conduct nondestructive tests in industry. The radiographic image detector converts radiation that has penetrated a subject into an electric signal to achieve radiographic imaging. The radiation here includes X-ray, α-ray, β-ray, γ-ray, electron beam, and ultraviolet ray.
Among known radiographic image detectors is a so-called flat panel detector (referred to as FPD below) that extracts an electric image signal from radiation.
There are two types of FPDs: a direct type and an indirect type. The direct type of FPD collects and reads out electron-hole pairs generated by a photoconductive film such as one formed of amorphous selenium in response to incident radiation, as an electric signal. Thus, the direct type directly converts radiation into an electric signal. The indirect type has a phosphor layer or a scintillator layer formed of a phosphor that emits light or fluoresces in response to incident radiation to convert radiation into visible light through that phosphor layer, reading out the visible light with a photoelectric transducer. Thus, the indirect type converts radiation into visible light and then the visible light into an electric signal.
There is also known an apparatus using, in lieu of an FPD, a radiographic image conversion panel or an imaging plate (IP) having a film formed of photostimulable phosphor that accumulates a part of the radiation energy in response to irradiation and emits photostimulable light corresponding to the accumulated energy in response to subsequent irradiation with an excitation light such as visible light. An example of such an apparatus in practical use is an FCR or Fuji Computed Radiography provided by FUJIFILM Corporation.
Where an FPD is used, linear unevenness (streak unevenness) containing streaks extending in the direction in which pixels are arranged may develop depending upon the imaging conditions under which a radiographic image is taken, and the linear unevenness (streak unevenness) thus developed varies according to the method used to read an electric signal corresponding to radiation received.
Further, when the line sensor has defective pixels, it produces streak unevenness in the scan direction as it scans and reads a radiographic image produced in the IP.
When using such an FPD or an IP to produce a radiographic image, there are cases where radiograph is achieved using a stationary grid having plates made of lead or another material that is impermeable to radiation and plates made of aluminum or another material that is permeable to radiation, alternately disposed therein, to prevent scatter light generated by a subject from entering an imaging medium. Use of such a stationary grid reduces generation of scatter light and thereby improves the contrast of a radiographic image but often causes the radiographic image to develop streak unevenness corresponding to edges of the lead plates used in the stationary grid.
Among methods of removing such streak unevenness from a radiographic image is one disclosed in JP 2003-150954 A filed by the Applicant of this application. According to this method, spatial frequency processing is performed using one-dimensional filters both in the same direction as that in which the streak unevenness (periodical pattern due to the stationary grid) extend and in the direction normal thereto to extract streak unevenness from the original image and subtract the streak unevenness from the original image, achieving removal of the streak unevenness from the original image.
For example, the original image is low-pass filtered one-dimensionally in the main scan direction, i.e., in the direction normal to the streak unevenness, to extract low-frequency components, which are then subtracted from the original image to extract high-frequency components, which in turn are subjected to one-dimensional low-pass filtering in the subscan direction to extract only the streak unevenness, which are then subtracted from the original image to remove the streak unevenness from the original image.
Thus, streak unevenness can be extracted and removed from the original image through the processing using a low-pass filter and a high-pass filter as described above.
However, filtering by a low-pass filter, etc. in the direction normal to the streak unevenness causes ringing in a region where a value of image data abruptly changes, such as at a streak, where the density greatly changes, or at the edge of an image.
For example, when an original image contains streak unevenness extending in the x direction as illustrated in FIG. 4A, a region A extending in the y direction normal to the streak unevenness and crossing one streak has a profile as indicated by a solid line in FIG. 4B.
Low-pass filtering the region A in the y direction normal to the streak unevenness results in obtaining an image having a profile as indicated by the broken line in FIG. 4B. If the low-pass filtered image (broken line) is subtracted from the original image (solid line), ringing will result, as illustrated in FIG. 4C, such that the streak will have an abnormally low density at both ends thereof, where artifacts are produced.
Further, according to the method of extracting streak unevenness by processing the original image using an image processed by, for example, a low-pass filter, should a subject be parallel to the streak unevenness, the subject may also be detected as a streak unevenness, so that removal of streak unevenness may entail eliminating the necessary subject from the original image.